Applications Information: continued
IC. This heat is transferred to the surface of the IC package,
TJ - TA
QJC + QCS + QSA
but a thermal gradient exists due to the thermal properties
of the package molding compound. The magnitude of this
thermal gradient is denoted in manufacturerÕs data sheets
as QJA , or junction-to-air thermal resistance. The on-chip
junction temperature can be calculated if QJA , the air tem-
perature at the ICÕs surface and the on-chip power
dissipation are known:
PD =
where PD is on-chip power dissipation in watts, TJ is junc-
tion temperature in ¡C, TA is ambient temperature in¡C,
and thermal impedance QJC , QCS , and QSA are in¡C/W.
All these quantities can be calculated or obtained from data
sheets. The choice of a heatsink is based on the value of
QSA required such that the calculated power dissipation
does not cause junction temperature to exceed the manu-
facturerÕs maximum specification.
TJ = TA + (QJA ´ P)
TJ and TA are given in degrees centigrade, P is IC power
dissipation in watts and QJA is thermal resistance in
degrees centigrade per watt. Junction temperature should
be calculated for all semiconductor devices to ensure they
are operated below the manufacturerÕs maximum junction
temperature specification. If any componentÕs temperature
exceeds the manufacturerÕs maximum specification, some
form of heatsink will be required.
EMI Management
Switching regulators generate noise a consequence of the
large values of current being switched on and off in normal
operation. Careful attention to layout of the printed circuit
board will usually minimize noise problems. Layout guide-
lines are provided in the next section. However, it may be
necessary in some cases to add filter inductors or bypass
capacitors to the circuitry to achieve the desired perfor-
mance.
Heatsinking will improve the thermal performance of any
IC. Adding a heatsink will reduce the magnitude of QJA by
providing a larger surface area for heat transfer to the sur-
rounding air. Typical heat sinking techniques include the
use of commercial heatsinks for devices in TO-220 pack-
ages, or printed circuit board techniques such as thermal
bias and large copper foil areas for surface mount pack-
ages.
Layout Considerations
The following guidelines should be observed in the layout
of PC boards for the CS5127:
When choosing a heatsink, it is important to break QJA
into several different components.
1. Connect the PGND lead to the external ground with a
wide metal trace.
QJA = QJC + QCS + QSA
2. Connect both LGND and PGND together with a wide
trace as close to the IC as possible.
where all components of QJA are given in ¡C/W.
QJC is the thermal impedance from the junction to the sur-
face of the package case. This parameter is also included in
manufacturerÕs data sheets. Its value is dependent on the
mold compound and lead frames used in assembly of the
semiconductor device in question.
3. Make all ground connections to a common ground
plane with as few interruptions as possible. Breaks in
the ground plane metal should be made parallel to an
imaginary line between the supply connections and the
load.
QCS is the thermal impedance from the surface of the case
to the heatsink. This component of the thermal impedance
can be modified by using thermal pads or thermal grease
between the case and the heat sink. These materials replace
the air gap normally found between heatsink and case with
a higher thermal conductivity path. Values of QCS are
found in catalogs published by manufacturers of heatsinks
and thermal compounds.
4. Connect the ground side of the COMP lead capacitors
back to LGND with separate traces.
5. Place the VFFB lead capacitors as close to the VFFB leads
as possible.
6. Place the 5V line bypass capacitors as close to the
switch FETs as possible.
7. Place the output capacitor network as close to the load
as possible.
Finally, QSA is the thermal impedance from the heatsink to
ambient temperature. QSA is the important parameter
when choosing a heatsink. Smaller values of QSA allow
higher power dissipation without exceeding the maximum
junction temperature of the semiconductor device. Values
of QSA are typically provided in catalogs published by
heatsink manufacturers.
8. Route the GATE lead signals to the FET gates with a
metal trace at least 0.025 inches wide.
9. Use wide straight metal traces to connect between the
5V line and FETs, between FETs and inductors and
between inductors and loads to minimize resistance in
the high current paths. Avoid sharp turns, loops and
long lengths.
The basic equation for selecting a heatsink is
14
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
